1. Field of the Invention
The present invention relates to a photoelectric conversion device and, more particularly, to a photoelectric conversion device which is suitably used in a radiophotographic apparatus such as a medical, digital X-ray imaging apparatus with a large area and high S/N characteristics.
2. Related Background Art
Not only in Japan in which the population of elderly people is increasing rapidly but also worldwide, improvement of diagnostic efficiency in hospitals and development of medical equipments with higher precision are strongly demanded. In such situation, an X-ray imaging apparatus using a film (film type apparatus) has been popularly used.
FIG. 1 is a schematic view showing the arrangement for explaining an example of a conventional film type X-ray imaging apparatus. In FIG. 1, an X-ray source 901 is arranged above an object 902 to be inspected (to be examined) such as a human body (patient), and a grid 903 is arranged beneath the object 902 to be inspected. The grid 903 is constituted by alternately arranging a substance that absorbs X-rays and a substance that transmits X-rays so as to increase the resolution. A scintillator (phosphor) 904 absorbs X-rays and emits visible rays. The visible rays emitted by the scintillator 904 are received by a film 905.
Such film type apparatus has the following problems.
Before a doctor acquires an X-ray image of a patient, a film development process must be performed, resulting in much labor and time.
Sometimes, when a patient moves during X-ray phototaking or when the exposure amount is improper, phototaking must be inevitably redone. These factors impede improving diagnostic efficiency in hospitals.
A clear X-ray image cannot often be obtained depending on the phototaking angle of the affected portion to be phototaken. For this reason, in order to obtain an X-ray image required for diagnosis, some images must be taken while changing the phototaking angle. Such operation is not preferred especially when the patient is an infant or a pregnant woman.
Furthermore, X-ray image films must be preserved after phototaking for a certain period of time in hospitals, and the number of such films becomes very large in hospitals, resulting in poor efficiency in terms of management in such institutions since the films must be put in and out every time a patient comes to a hospital.
When a patient needs to change the hospital he or she normally visits to seek medical attention for some reason, for example, when a patient in a remote place must undergo diagnosis as highly advanced as that he or she can receive only in a midtown university hospital or must move abroad, X-ray films after exposure and development must be delivered to the next hospital by some method. Otherwise, the patient must be subjected to phototaking again in the new hospital. These problems are serious obstacles against establishing a new system of medical practice in future.
In recent years, in medical industries, demand for "digitization of X-ray image information" is increasing. If the digitization is attained, X-ray image information can be managed using recording media such as magneto-optical disks, and a doctor can acquire X-ray image information at an optimal angle in real time. When communication systems such as a facsimile system, and the like are utilized, X-ray image information can be sent to hospitals everywhere in the world within a short period of time. Furthermore, when the obtained digital X-ray image information is subjected to image processing using a computer, diagnosis with higher precision than in the conventional method can be realized, and all the problems that the conventional film method has encountered can be solved.
Recently, an X-ray imaging apparatus that uses a CCD solid-state imaging element in place of a film has also been proposed to meet demand for "digitization of X-ray image information". For this reason, when a CCD solid-state imaging element is used, fluorescence, i.e., an X-ray image, from the scintillator must be imaged on the CCD light-receiving surface via a reduction optical system. This poses a problem of an increase in scale of the X-ray imaging apparatus. On the other hand, since an X-ray image is formed via a lens, it is generally accepted that the S/N (signal to noise) ratio is reduced by two to three orders upon passing through the lens, and this fact is expected to be disadvantageous upon applying the CCD solid-state imaging element to medical equipment that require high gradation characteristics.
In recent years, upon development of photoelectric conversion semiconductor thin films represented by hydrogenated amorphous silicon (to be abbreviated as a-Si hereinafter), so-called contact sensors which are constituted by forming photoelectric conversion elements on a large-area substrate and can attain reading by an optical system at an equal magnification to an information source have been developed extensively. In particular, since a-Si can be used not only as a photoelectric conversion material but also a thin film field effect transistor (to be abbreviated as a TFT hereinafter), photoelectric conversion semiconductors and a semiconductor layer of TFTs can be simultaneously formed on a single substrate. Since the surface area can be increased so that an image can be read at an equal magnification without using a reduction magnification system, the S/N ratio can be higher than that of the CCD solid-state imaging element. In addition, since the necessity of the reduction optical system can be obviated, a size reduction of the apparatus can be promoted, and such apparatus is effective for a small medical institution that cannot assure a large space, a diagnosis vehicle that carries an X-ray imaging apparatus, and the like. Owing to these merits, X-ray imaging apparatuses using an a-Si semiconductor thin film have been extensively developed. More specifically, an X-ray imaging apparatus in which photoelectric conversion elements and TFTs using the a-Si semiconductor thin film replace the film portion 905 in FIG. 1, and which electrically reads an X-ray, image has been developed.
The X-ray dose on human bodies has an upper limit in hospitals although it varies depending on the affected portions. In particular, in diagnosing an infant or a pregnant woman, the dose must be reduced as much as possible. Therefore, in general, the light-emission amount of a scintillator (phosphor) that absorbs X-rays and converts them into visible rays, and the charge amount in an a-Si photoelectric conversion element which receives fluorescence and photoelectrically converts it are small. In order to obtain a clear image from a weak signal, wiring lines must be shortened as much as possible so as to prevent noise components from superposing on analog signal wiring lines extending from a photoelectric conversion panel, and an analog signal must be received by a buffer amplifier to decrease the impedance. Furthermore, in order to eliminate the influence of noise, the analog signal is preferably A/D-converted in the vicinity of the buffer amplifier to store digital data in a memory.
When a digital X-ray imaging apparatus is constituted using a photoelectric conversion panel on which photoelectric conversion elements having an a-Si semiconductor thin film are arranged two-dimensionally, it is generally accepted that the pixel pitch is preferably set to be 100 .mu.m or less in terms of resolution. Also, for chest x-rays of a person, it is generally accepted that the effective pixel area of the photoelectric conversion elements preferably has at least a size of 400 mm.times.400 mm. When a photoelectric conversion panel having an effective area of 400 mm.times.400 mm is formed at 100-.mu.m pitch, the number of pixels is as large as 16 million. When photoelectric conversion signals from such large number of pixels are processed, buffer amplifier ICs and A/D-conversion ICs must operate at high speed. In particular, when moving images are to be phototaken, higher-speed processing is required, and each IC requires large consumption power. When a large volume of digital data are to be transmitted to a remote place outside an X-ray phototaking room at high speed, since a high-speed type line driver required for removing transmission errors comprises an IC mainly constituted by bipolar transistors, it requires larger consumption power as higher-speed specifications are attained, and becomes an insignificant heat generation source.
In recent years, high-speed CMOS-ICs with small consumption power have been developed remarkably, and their further advance in future is expected. However, as far as the versatile ICs are concerned, the performance of such CMOS-IC cannot compare with that of ICs using bipolar transistors. As a consequence, ICs mainly constituted by high-speed bipolar transistors must be used, and heat produced by ICs themselves upon increase in consumption power has a serious influence on an X-ray imaging apparatus.
Heat produced by an IC raises the temperatures of a-Si photoelectric conversion elements and TFTs in the X-ray imaging apparatus. In general, dark currents and photocurrents in a-Si photoelectric conversion elements change in correspondence with the temperature rise. Since changes in dark current produce temperature differences in the two-dimensional array of photoelectric conversion elements, dark currents may vary in the plane to impose an adverse influence in the form of fixed pattern noise (FPN). Also, shot noise in the photoelectric conversion elements may impose an adverse influence in the form of random noise (RDN). Furthermore, the temperature unevenness of the photoelectric conversion elements upon reading may induce in-plane shading in the output. Moreover, so-called KTC noise (K: Boltzmann's constant, T: absolute temperature, C: capacity in transfer system) is produced upon transferring accumulated signal charges from the photoelectric conversion elements, and may inflict an adverse influence (RDN). The above-mentioned temperature rise of the photoelectric conversion elements and TFT induces a decrease in S/N ratio of the X-ray imaging apparatus and variations in S/N ratio among pixels, thus deteriorating image quality. In addition, the reliability of the apparatus may be impaired.
However, since all the X-rays are not always converted into visible rays in the phosphor, some scattered or transmitted X-rays are radiated onto the above-mentioned buffer amplifier, memory, or other digital ICs in the vicinity of the photoelectric conversion panel. Such X-rays deteriorate the performance of ICs formed by crystalline Si, and the apparatus may malfunction over a long term of use, thus posing the problem of reliability. For this reason, in addition to the above-mentioned problems, it is desired to take a measure against exposure of unwanted portions to X-ray radiation.
Such problems may be posed not only in the photoelectric conversion device used in the X-ray imaging apparatus but also in a large-area, multiple-pixel photoelectric conversion device which can convert light information into electrical information.
Also, similar problems may be posed not only in a photoelectric conversion device for an imaging apparatus which uses radiation such as X-rays as a light source, but also in a photoelectric conversion device which is used in non-destructive inspections to realize high-speed processing and a high-resolution, large-area structure.